A multi-phase ac spindle motor (generally, a three-phase ac spindle motor) has widely been employed for driving the spindle unit of a machine tool, i.e., the output shaft of such a multi-phase ac spindle motor has an end adapted to drive the spindle.
A multi-phase ac spindle motor of such a kind is incorporated into recent numerically controlled machine tools equipped with an automatic tool changer and machining centers, and the numerically controlled machine tools and the machining centers have advanced mechanisms for rotating the spindle at a high spindle speed. The numerically controlled machine tools and the machining centers, however, have been required to be capable of satisfactory cutting and grinding when the spindle is rotated at a spindle speed in a low-speed range as well as in a high-speed range. To meet such a requirement, the ac spindle motor must be capable of stably rotating while developing a predetermined torque in a wide speed range including a relatively low rotating speed to a relatively high rotating speed. A spindle motor for such purposes has been proposed in, for example, Japanese Patent Application No. 63-144851 filed previously by the applicant of the present application. This previously proposed spindle motor secures a stable output torque over a wide speed range by increasing or decreasing the effective number of turns of the excitation windings by changing the connection of the windings in varying the spindle speed in a wide speed range from a relatively low spindle speed to a relatively high spindle speed by varying the number of the frequency of an electric current for generating a revolving magnetic field.
A 4-pole ac induction motor has satisfactory output torque characteristics in a low-speed range, but the current control system of a driver for driving the 4-pole ac induction motor must be changed from an asynchronous current control system to a synchronous current control system for operation in a high-speed range, which requires a troublesome procedure. Accordingly, in most cases, a 2-pole ac induction motor having satisfactory output characteristics in a high-frequency range is employed. The thickness and diameter of each core plate, i.e., a core lamination, of the laminated cores of the 2-pole and 4-pole motors are determined properly so that the laminated core has desired magnetic characteristics according to the number of magnetic poles.
Accordingly, it has further been desired to provide an ac spindle motor incorporating a multi-phase ac induction motor capable of effectively utilizing the magnetic characteristics of the laminated core and output characteristics dependent on the number of magnetic poles.
In utilizing the characteristics of a laminated core of a small thickness and a small core diameter incorporated into a conventional 4-pole ac induction motor, the inherent satisfactory speed-torque characteristics of the 4-pole ac induction motor are effective in a low-speed range. In a high-speed range, however, it is difficult to increase the rotating speed of the 4-pole ac induction motor to a sufficiently high rotating speed by increasing the number of frequencies without changing the number of magnetic poles because an unlimited increase in the frequency number is difficult owing to restrictive conditions on the driver. On the other hand, since the diameter of the core yoke of a laminated core for a conventional 2-pole ac induction motor, in general, is greater than that of a laminated core for a 4-pole ac induction motor, employment of a laminated core for a conventional 2-pole ac induction motor inevitably increases the size of the ac spindle motor.